EP 0,349,420 discloses a method with an apparatus for the preparation and/or coating of the surfaces of metallic hollow structural elements, which comprise at least two connection openings between their outer and inner surfaces, especially for hollow blades in the field of turbine engine construction. In the disclosed method and apparatus, a cleaning gas mixture or a coating gas mixture is generated beneath a blade in a reaction space. The blade hangs in the reaction space, from which the outer surfaces may be cleaned or coated. The reaction gas first flows over the outer surfaces in one direction and then flows through a first opening in the hollow blade into the hollow spaces and past the inner surfaces, and finally flows out of the hollow spaces through a second opening in the hollow blade into an exhaust conduit for removal or return flow of the residual gases of the reaction gas.
Such apparatus and methods have the disadvantage that the concentration of individual reaction components, which are contained in the reaction gas and which react with the surfaces, diminishes along the path over the outer surfaces, the first opening, the inner surfaces, and up to the outlet out of the second opening, to such an extent that substantial reaction differences arise between the outer and inner surfaces and over the course of the inner surfaces.
The reaction differences between the outer and inner surfaces are partially overcome by measures as are described in the patents DE 4,035,789 and DE 4,119,967. However, it can be determined that the differences over the course of the inner surfaces from the entry into the hollow spaces up to the point of flowing out of the hollow spaces cannot be substantially improved using the prior methods. Moreover, the improved methods and apparatus have the disadvantage that they require retort structures that are constructed in an extremely complex and only slightly variable manner, and appear to be unsuitable for use in mass production.
A further essential disadvantage of the prior art is that the known apparatus and methods do not permit the use of different gas sources for the treatment of the outer and inner surfaces.